Coupled modelling for dynamic submarine power cables: interface sensitivity analysis of global response and local structural engineering models

Abstract

Conventional power transmission cables have been designed to operate in conditions that differ considerably from those experienced by cables servicing floating marine renewable energy converters. Cables attached to floating platforms are subject to greater levels of mechanical and electrical stress due to the motion of the platform in a highly energetic offshore environment and are termed dynamic cables. The combination of the different loads from waves, wind and currents, in shallower waters are complex and need to be assessed through a combination of coupled numerical models and experimental tests key to this is the verification of fatigue strength of the cable. A dynamic power cable is susceptible to fatigue or over bending when there is an abrupt change in bending stiffness. The electrical capabilities of subsea cables are well understood; however, the structural capabilities are not. Previous work has shown that the local effects in the cable cross-section play an important role in the assessment of fatigue life. A good understanding of the internal structure of a subsea cable, and interaction between the layers, is integral to the development of robust and reliable, high voltage, dynamic, subsea cables. Global analyses, assessing the overall motions of the floating platform, moorings and cables, are carried out to provide data to inform the cable design process. Such analyses are highly dependent on the input of local structural response coefficients which are available only through detailed local structural analysis numerically and/or experimentally. There is a strong need to gain a better understanding of the local structural assessment of cable cross-sections and the coupling of the data attained through the local assessments with the global modal. This work builds on previous experience in fatigue assessment in subsea cables with an OrcaFlex global model of a generic floating wave energy converter platform and moorings. The study will incorporate a dynamic subsea cable with detailed local properties, derived through numerical local structural analysis. The results will show the sensitivity of the global analysis to the locally determined structural results, for example regarding bend stiffness and tensile stiffness.